1. Field of the Invention
The present invention relates, in general, to a PLL (phase-locked loop) circuit which is used to improve the C/N ratio of a broad-band voltage controlled oscillator (hereinafter, referred to as "a VCO" for brevity, when applicable) which has high sensitivity, and more particularly to a PLL circuit which is utilized in a synthesizer for which an excellent spectrum is required, and multi-channel FM receiving method and apparatus which are capable of utilizing this PLL circuit and which are utilized as either a front-end device of a BS or CS tuner, or a receiving side front-end device used in CATV service and the like in a local area.
2. Description of the Related Art
Heretofore, in such a PLL circuit, for example, a digital phase comparator has been used and its phase detecting voltage is subjected to amplification and filtering through a charge pump circuit to be used as a control voltage for a VCO. FIG. 1 is a functional block diagram showing a configuration of a conventional digital PLL circuit. In FIG. 1, the reference numeral 1 designates a digital phase comparator which operates to compare two digital signals input thereto to output a signal representing a phase difference therebetween, and the reference numeral 2 designates a charge pump which operates to smooth the phase difference signal through an active type low-pass filter, thereby subjecting the pulse waveform to amplification and smoothing. The reference numeral 3 designates a VCO which receives a control voltage as a DC voltage to vary a frequency of an output oscillation signal. The output signal of the charge pump 2 is supplied as the control voltage to the VCO 3. The reference numeral 4 designates a pre-scaler which frequency-divides the output oscillation signal of the VCO 3 to obtain a feedback signal having a suitable frequency, and the reference numeral 5 designates a reference signal generator which generates a reference signal.
In the configuration shown in FIG. 1, the digital PLL circuit is designed in such a way that the phase error occurring between the reference signal and the feedback signal is detected by the digital phase comparator 1, and the DC control voltage corresponding to that phase error is input to the VCO 3 through the charge pump 2, thereby obtaining an output signal having a predetermined frequency.
On the other hand, FIG. 2 is a functional block diagram showing a configuration of a conventional analog PLL circuit. In FIG. 2, the reference numeral 11 designates a multiplier (a mixer) which multiplies two analog signals, which are different in phase from each other, thereby outputting a signal representing the phase difference between the two signals. The reference numeral 12 designates a passive type low-pass filter which eliminates the high frequency components. The reference numeral 13 designates a VCO which oscillates in accordance with a control voltage input thereto. The reference numeral 14 designates a prescaler which receives a tuning voltage to divide the output signal of the VCO, thereby producing an analog feedback signal, and the reference numeral 15 designates a reference signal oscillator which generates a reference signal.
In the configuration shown in FIG. 2, the mixer 11 multiplies the analog reference signal which has been output from the reference signal generator 15 by the analog feedback signal which has been obtained by frequency-dividing in the pre-scaler 14 and then outputs a signal which includes a phase error signal corresponding to the phase difference therebetween, and a phase summing signal having a frequency two times as high as that of the input signal. Then, the output signal of the mixer 11 is supplied to the low-pass filter 12 in which both the phase summing signal and the remaining carrier component are eliminated, and as a result, the DC control voltage corresponding to the phase error is output from the low-pass filter 12. In addition, the control voltage is supplied to the VCO 13. Thus, the analog PLL circuit is constructed in this manner.
However, in the above-mentioned conventional PLL circuits, each of the digital phase-comparison type and the analog phase-comparison type had its merits and demerits from a viewpoint of the synthesizer control of the broad-band VCO having high sensitivity. As a result, it was impossible to fulfill both the aspects of the good C/N ratio improvement and the broad-band lock. That is, there arose the problem that a sufficient C/N ratio or noise suppression could not be obtained in the digital PLL circuit. On the other hand, in the analog PLL circuit, since the amplitude of the output voltage of the mixer acting as the phase comparator was small and also the sine-wave analog signal was input to the mixer, it was difficult to obtain the broad-band drawing state (the lock state).
Next, a description will hereinbelow be given with respect to conventional FM receiving method and apparatus utilizing the PLL circuit of this kind as described above.
Heretofore, the FM receiving method and apparatus of this kind have utilized the down-converting system. FIG. 3 is a functional block diagram showing a configuration of the conventional multi-channel FM receiver. In FIG. 3, the reference numeral 21 designates a high-pass filter which operates to select a multi-channel FM signal from an input signal, and the reference numeral 22 designates a gain variable control unit which adjusts the amplitude of the FM signal which has been obtained from the high-pass filter 21 to a predetermined level. In addition, the reference numeral 23 designates a variable band-pass filter which operates to select the FM signal of a desired channel in accordance with a channel selection command issued from a controller (not shown). The reference numeral 24 designates a broad-band amplifier which amplifies the selected FM signal. The reference numeral 25 designates a converting mixer which receives a local oscillation signal to down-convert the FM signal obtained from the amplifier 24 and to produce a signal having an intermediate frequency. Further, the reference numeral 26 designates a SAW (Surface Acoustic Wave) filter which eliminates the unnecessary components of the intermediate frequency signal obtained from the down-converting mixer 25.
The reference numeral 27 designates a VCO, an oscillation frequency of the output signal of which is changed in accordance with a DC control signal. The reference numeral 28 designates a distributor which receives the output signal supplied from the VCO 27 as an input signal to distribute the input signal into the local oscillation signal which is supplied to the mixer 25 and a feedback signal which is used to produce the DC control signal. The reference numeral 29 designates a pre-scaler which operates to frequency-divide the feedback signal supplied thereto by 1/128. The reference numeral 30 designates a programmable pre-scaler which operates to further frequency-divide the feedback signal, which has been obtained by the frequency-dividing in the pre-scaler 29, by 1/N (N is a positive integral number and changes depending on a channel selection signal) in accordance with the channel selection command issued from the controller (in particular, not shown).
The reference numeral 31 designates a reference signal generator which generates a reference signal. The reference numeral 32 designates a phase error detecting circuit which detects a phase error occurring between the feedback signal obtained from the programmable pre-scaler 30 and the reference signal to produce a phase error signal. The reference numeral 33 designates an amplifier which amplifies the phase error signal, and the reference numeral 34 designates a low-pass filter which extracts the DC component from the phase error signal to supply the DC component as the DC control signal for the VCO 27. By adopting the configuration shown in FIG. 3, the receiving operation according to the single super receiving method can be performed.
Next, a description will hereinbelow be given with respect to the operation of the conventional multi-channel FM receiver. Now, it is assumed that the multi-channel FM signal, a frequency of which is in the range of about 950 MHz to 2,000 MHz according to the BS (Broadcasting Satellite) format, is received as the input signal. The multi-channel FM signal thus input is eliminated with the unnecessary low frequency components thereof through the high-pass filter 21, and then adjusted to the predetermined level by the gain variable control unit 22. Then, only the desired channel signal is selected by the variable frequency band-pass filter 23, thereby suppressing the image disturbance. Thereafter, the FM signal of the selected channel is amplified by the broad-band amplifier 24 and then is converted into the intermediate frequency signal having a frequency of 402.78 MHz by the down-converting mixer 25 to pass through the SAW filter 26. Then, the resultant signal is detected by a detector (in particular, not shown).
In addition, the local oscillation signal which is used in the down-converting operation is made oscillate at the frequencies of about 1,350 MHz to 2,400 MHz, and the oscillation signal is controlled by using the comparison frequency of about 10 kHz in the PLL synthesizer. That is, the loop circuit extending from the VCO 27 up to the low-pass filter 34 constitutes the PLL circuit 35 which is controlled by the controller (in particular, not shown).
However, in the above-mentioned conventional single super-receiving method, in the case where the number of channels increases, the variable frequency range of the variable frequency band-pass filter 23 becomes wide and thus it is difficult to put the variable frequency range band-pass filter 23 into practice. This is a problem. In addition, the oscillation frequency band of the VCO 28 requires necessarily the range exceeding one octave. In this case as well, it is impossible to prevent the circuit from being complicated. This is another problem. In addition, since the local oscillation signal is present in the frequency band of the input signal, in the case where a plurality of tuners are driven in parallel with one another, the leakage of the local oscillation signal hinders the operation of the adjacent tuners. This is still another problem.